The present invention relates to a new and improved method of, and apparatus for, reporting unauthorized intrusions by means of a sonic or acoustical receiver or transducer responsive to sounds transmitted through solids and mounted at a region or surface which is to be protected, there also being provided an electric circuit connected with the sonic receiver for producing a signal when the received sonic oscillations or vibrations exceed predetermined values.
Such systems are used to detect characteristic sonic oscillations or vibrations which are produced when there is attempted an unauthorized intrusion, typically a burglary, for instance by penetrating a wall or concrete structure, upon cutting or welding open a metal wall, for instance a vault or a door, and for evaluating the given alarm signals. In order to avoid tripping a faulty alarm it is, however, necessary that such systems exclusively respond to the oscillations produced solely by the intrusion attempt, for instance to the vibrations brought about by the breaking open or drilling or any otherwise unauthorized penetration of a protective wall or structure or the sonic oscillations or vibrations produced for instance with an oxyacetylene torch or an oxygen lance during the welding cutting of a metal or concrete wall, not however to external or ambient spurious noises.
It is already known to the art to eliminate the influence of the external spurious noises which are transmitted by the air by sound insulation of the sonic receiver.
In order to eliminate the external spurious noises which are transmitted by the wall or by the structure itself there is exploited the fact that the sonic oscillations produced when there is an intrusion attempt lie in a characteristic frequency range, preferably in the kHz-range near to the upper audio threshold, whereas spurious noises are usually of low frequency. An arrangement operating according to this principle, has been disclosed for instance in U.S. Pat. No. 3,134,970 and employs for the evaluation of the sonic oscillations of spurious noises, received by a piezoelectric receiver, a high-pass filter, thereby eliminating the low-frequency constituents. In order to eliminate briefly lasting spurious noises the output signal of the high-pass filter is integrated, so that an alarm signal is only triggered when there prevails sufficiently long lasting high-frequency sonic oscillations.
To ensure that such systems respond not only to such sonic oscillations, but also to a sudden attack at the region or surface to be protected, for instance by means of an explosive, there is taught to the art an arrangement in U.S. Pat. No. 3,147,467 where there is provided an additional evaluation channel which, upon occurence of very pronounced vibrations of short duration, likewise trips an alarm signal. The threshold value of this additional channel is chosen to be so high that in the presence of an accidental contact with the wall there is still not tripped an alarm signal, but in the presence of hard impact or an explosion attempt there will be immediately tripped an alarm, i.e. only with a slight time delay. Consequently, the susceptibility to disturbances of intrusion reporting arrangements working with sonic receivers is clearly reduced. However, what is disadvantageous with this system design is that with the use of high-pass filters or amplifiers the low-frequency spurious noises, which are better propagated in most walls or structures, can only be poorly segregated from high-frequency oscillations which are predicted upon an intrusion attempt. This leads to the result that weak intrusion vibrations at large distance from the receiver have superimposed thereon spurious noises, so that it is necessary to provide a large number of sonic receivers at a slight spacing from one another in order to afford adequate protection.
From U.S. Pat. No. 3,471,846 there is known to the art an intrusion detection system wherein there is obtained a sharply limited frequency range in that, the feeler signal is mixed with a carrier frequency which periodically continuously passes through a frequency range, and the mixed signal passes through a narrow band filter and is integrated over a number of throughpass periods. The installation indeed allows the elimination of a certain constant oscillation, but however is afflicted with the disadvantage that periodic spurious oscillations, which accidentally lie in the frequency throughpass range, likewise will trigger an alarm and not only the sonic oscillations or vibrations which emanate from an intruder.